1. Technical Field
The invention relates generally to roller cone drill bits used to drill wellbores through earth formations. More specifically, the invention relates to particular structures for roller cone drill bits having increased drilling efficiency and life expectancy.
2. Background Art
Roller cone drill bits are commonly used for drilling wells in the petroleum industry. FIG. 1 illustrates one example of a roller cone bit used in a conventional drilling system for drilling a well bore in an earth formation. The drilling system include a drilling rig 10 used to turn a drill string 12 which extends downward into a wellbore 14. Connected to one end of the drill string 12 is a roller cone drill bit 20.
As shown in FIG. 2, the roller cone bit 20 includes a bit body 22 having an externally threaded connection 24 at one end, and a plurality of roller cones 26 attached at the other end of the bit body 22 and able to rotate with respect to the bit body 22. FIG. 2 shows the roller cone bit 20 having three roller cones 26 which is the number typically used in the prior art. Attached to the roller cones 26 are a plurality of cutting elements 28 typically arranged in rows about the surface of the roller cones 26. The cutting elements 28 may be formed from a variety of substances, including tungsten carbide inserts, polycrystalline diamond inserts, boron nitride inserts, milled steel teeth, or other suitable materials. Additionally, the cutting elements 28 may be coated with a hardfacing material (not shown) to enhance the durability of the cutting elements 28.
Typically, prior art drill bits have been designed through a trial and error process that included selecting an initial design, field testing the initial design, and then modifying the design to improve drilling performance. Prior art bits used for soft to medium hardness formations typically have at least one roller cone with a xe2x80x9cstaggeredxe2x80x9d row of cutting elements on that cone. FIG. 3A illustrates what is meant by a staggered row. FIG. 3A shows an oblique view of a single roller cone 30 which includes a gage row of cutting elements 31. The gage row of cutting elements 31 functions to cut the borehole sidewall, borehole corner, and borehole bottom. An adjacent row of cutting elements 32, located immediately laterally interior of the gage row of cutting elements 31 is known as a xe2x80x9cstaggeredxe2x80x9d row because the adjacent row cutting elements 32 are not directly azimuthally aligned with and are spaced between the gage row cutting elements 31 as to their positions about the circumference of the roller cone 30. Further, as shown in FIG. 3A, the immediately interior (adjacent) row cutting elements 32 partially overlap the gage row cutting elements 31 in cross section profile. Thus, the immediately interior row of cutting elements has a cutting element count that is dependent on the gage row count.
Second interior row cutting elements 33, and third interior row cutting elements 34 on the roller cone shown in FIG. 3A are generally (azimuthally) rotationally aligned independently of the gage row cutting elements 31 and, thus are described as being non-staggered.
Prior art roller cone drill bits typically included staggered rows of cutting elements similar to the ones shown in FIG. 3A on only one or two of the roller cones. More specifically, for drill bits in which the cutting elements are tungsten carbide inserts, the structure of typical prior art bits can be categorized as follows. The drilling industry categorizes roller cone drill bits according to the types of earth formations which the bit is particularly designed to drill efficiently. One such categorization is known as the IADC (International Association of Drilling Contractors) Class of the bit. Examples of drill bits classified in certain IADC classes are shown in FIG. 3B. Drill bits classified in IADC series 5, 6, 7 and 8 in the prior art only included staggered rows on two of the three roller cones.
Another type of prior art drill bit using tungsten carbide inserts is shown in FIG. 4. Typically, as the formations for which the bit is designed become progressively harder, the cutting elements become relatively shorter with respect to their extension length from the surface of the roller cone. Cutting element extension length to diameter ratios for bits in the previously described IADC series 5, 6, 7 and 8 are shown as clusters of points for each such bit in each series.
Some bits, for example as shown at 41, 42, 43 and 44 in FIG. 4, have insert extension length to diameter ratios which exceed about 0.829. All of these bits in the prior art have a staggered row of cutting elements on only one roller cone.
One reason that prior art bits included only one or two cones having staggered rows is that it was generally believed that the number of cutting elements in the gage rows needed to be kept relatively high to increase durability, so that the bit would drill a substantially gage hole for a long as possible during the useful life of the bit. The geometry of the bits necessitated having only one or two cones with staggered rows of cutting elements in order to maximize to the greatest extent possible the cutting element count in the gage rows. Having bits with only one or two cones with staggered rows of cutting elements can reduce the effective life and drilling performance of such drill bits.
One aspect of the invention is a roller cone drill bit including a bit body having three legs depending therefrom, each leg having a journal on it. A roller cone is rotatably mounted on each journal, and each roller cone has a plurality of cutting elements on it. The cutting elements are arranged in rows on each roller cone. The rows of cutting elements include at least a gage row and a first row interior of the gage row. The first interior row is staggered with respect to the gage row on each one of the three cones. In one embodiment, the cutting elements are tungsten carbide inserts.
In another embodiment, a maximum distance between the cutting elements on the gage row and the first interior row on any one of the cones is about zero to 350 percent greater than a minimum distance between the cutting elements on the gage row and the first interior row on any other one of the cones. In another embodiment, an area of the hole bottom surface cut by the cutting elements on the gage row and the first interior row on the three roller cones is between about 20 and 60 percent of a total area of the hole bottom cut by all the cutting elements on all the roller cones.
Another aspect of the invention is a roller cone drill bit including a bit body having three legs depending therefrom. Each leg has a journal, a roller cone rotatably mounted on each journal, and each roller cone has a plurality of cutting elements thereon. The cutting elements are arranged in rows on each cone. The rows include at least a gage row and a first row interior of the gage row. The first interior row is staggered with respect to the gage row on at least two of the three cones. The cutting elements have an extension to diameter ratio of at least 0.829.
In one embodiment, a maximum distance between the cutting elements on the gage row and the first interior row on any one of the at least two cones is about zero to 350 percent greater than a minimum distance between the cutting elements on the gage row and the first interior row on any other one of the at least two cones. In another embodiment, an area of a hole bottom surface cut by the cutting elements on the gage row and the first interior row on the three roller cones is between about 20 and 60 percent of a total area of the hole bottom cut by all the cutting elements on all the roller cones.
Another aspect of the invention is a structure for a roller cone drill bit. The bit includes a bit body having three legs depending therefrom, each leg having thereon a journal, a roller cone rotatably mounted on each journal, and each roller cone has a plurality of cutting elements thereon. The cutting elements are arranged in rows on each cone, the rows including at least a gage row and a first row interior of the gage row. A fractional amount of a hole bottom area defined by the cutting elements in the gage row and the first interior row is less than a boundary amount defined with respect to a cutting element extension to diameter ratio, the boundary amount substantially conforming to values of the ratio 0.330, 0.436, 0.49, 0.52, 0.56, 0.592 0.658, 0.688, 0.747, 0.769, 0.8, 0.875, 1.04 and corresponding fractional amounts of 42.30, 36.44, 48.32, 50.67, 50.65, 45.3, 51.80, 49.53, 54.45, 58.08, 60.88, 61.81, and 65.57 percent.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.